Approach control apparatus for railway signaling systems



Jan. 1941- c. E. STAPLES 2,228,792

APPROACH CONTROL APPARATUS FOR RAILWAY SIGNALING SYSTEMS Filed July 19, 1958 Patented Jan. 14, 1941 UNITED STATES APPROACH CONTROL APPARATUS FOR RAILWAY SIGNALING SYSTEMS Crawford E. Staples, Wilkinsburg, Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application July 19, 1938, Serial N0. 220,015

8 Claims.

'My invention relates to approach control apparatus for use in railway signaling systems of the coded track circuit class and it has special reference to the employment of such apparatus for approach controlling various signaling functions without the use of line wires.

Generally stated, the object of my invention is to improve certain features of an approach control without line wire scheme wherein the rails 10 of each unoccupied signal block length of track transmit code step pulses of auxiliary energy forwardly from the block entrance to efiect the energization of a slow release approach relay at the block exit.

of the referred to pulses of auxiliary energy to the entrance end of each track circuit without interfering with the normal code following operation of the signaling system track relay at that location.

Another object is to facilitate the detection of these pulses at the exit end of each track circuit and to provide improved means for energizing the associated slow release approach relay in step with them.

An additional object is to insure that this relay energization will be effected without interference from the coded signal control energy which facilities of the signaling system supply to the track circuit at the location of the approach relay.

A still further object is to provide cut section facilities which are suitable for use with approach control schemes of the improved type herein disclosed.

In practicing my invention, I attain the above and other objects and advantages by introducing the pulses of auxiliary energy into the track circuit at times when the track relay is picked up; by employing auxiliary energy having a voltage which is much higher than that of the energy of the main signal control code; by bridging directly across the rails at the exit end of the track circuit a sensitive code following detector relay which responds only to potential of the high auxiliary energy value and which is unaffected by the coded signal control energy; and by energizing the associated slow release approach relay in step with the responses of this detector relay.

I shall describe a few forms of approach control apparatus embodying my invention and shall their point out the novel features thereof in claims. These illustrative embodiments are disclosed in the accompanying drawing in which:

Fig. l is a diagrammatic representation of a A more specific object is to facilitate the supply section of railway track which is equipped with one preferred form of the approach control apparatus of my invention;

Fig. 2 is a similar showing of a modified form of entrance end apparatus which may be substituted for the entrance end facilities of Fig. 1; and

Fig. 3 is a diagrammatic view of cut section facilities which are suitable for use with the approach control schemes of Figs. 1 and 2. m

In the several views of the drawing like reference characters designate corresponding parts. Referring first to Fig. 1, the improved approach control apparatus of my invention is there disclosed in association with a coded track circuit system of automatic block signaling for a railway track l2 over which it will be assumed that trafiic moves in the single direction indicated by the arrow, or from left to right in the diagram. The protected stretch of this track is divided into the customary successive sections by insulated rail joints 3 and the rails of each section form a partof a track circuit to which coded signal control energy is supplied in customary manner. 25

In this view of Fig. 1, reference characters D and E respectively designate the entrance and the exit ends of one of these track sections which is illustratively shown as being a full signal block in length; character TR designates a code 30 following track relay which is installed at the entrance of the section and operated by energy received from the railsv thereof; character TB a track battery or other direct current source provided at the section exit for the purpose of supplying the rails with the relay operating energy just referred to; character OR a coding device having a contact 5 which codes this energy by periodically interrupting the rail supply circuit; and character S the usual wayside 40 signal which guards the entrance of each of the track blocks and which is controlled by the associated track relay TR through the medium of decoding apparatus l0.

An automatic block signaling system of the referred to coded track circuit type operates without the aid-of line wires and includes all of the elements just named. Such a system further comprises: the customary facilities (not shown) for continuously operating each of the exit end relays OR at one or another of the usual plurality of distinctive code rates. Selection among these rates (which in a typical three-indication system may consist of and energy pulses per minute) is made in accordance with advance traflic 55 conditions by the decoding apparatus Ill functioning in customary manner.

This decoding apparatus (details not shown) is controlled in the usual fashion by the associated track relay TR and it performs the further function of selectively setting up a lighting circuit for one or another of the lamps (G, Y and R in the typical three-indication system above referred to) of the wayside signal S at the same location. In the arrangement represented, these signal lamps derive energizing current from a power source which is designated by the terminals plus and minus.

For applications in which train carried cab signals (not shown) also are to be controlled, the representative coded track circuit signaling facilities still further comprise means at the exit end of each track section for additionally supplying the rails thereof with coded alternating current energy. In the form shown at location E in Fig. 1, these means include a track transformer TT having a secondary winding which is bridged across the track rails by coding contact 5 of device CR during each energy off period of the main track circuit code (from battery TB) and a primary winding which at proper times is energized over a conductor 1 from a suitable alternating current source designated by the ter minals B and C.

In order that certain functions of the signaling system may be rendered active only upon the approach of a train, the apparatus installed at each of the signal locations D, E, etc. further includes an approach relay AR which is arranged to maintain the referred to functions inactive at all times except when the section of track to the rear of the location becomes occupied. In the illustrative arrangement shown at location E in Fig. 1, these approach controlled functions consist in lighting the wayside signal Se (normally dark) and in supplying the rails of the section to the rear of that signal with alternating current energy for cab signal control.

The former function is governed by a contact 9 of the approach relay AR and the latter by a companion contact I l thereof. Either of these functions may, of course, be controlled individually by the approach relay and it will be apparent, moreover, that signaling functions other than or in addition to those just named may likewise be governed by the same relay.

When applied to coded signaling systems of the conventional character just considered, the improved approach control apparatus herein disclosed renders the relay AR at each signal location responsive to the approach of a train and does this, moreover, without the use of control line wires.

For each of the signal blocks my improved apparatus comprises: (1) entrance end facilities which supply the rails of that block with pulses of auxiliary energy which are in step with recurring periods of the main track circuit code and (2) exit end facilities which receive their auxiliary pulses from the rails and energize the approach relay AR in step with them.

In the form shown at location D in Fig. 1, my improved entrance end facilities include an impulse transformer I'I which is connected in energy supplying relation with the rails I and 2 of the track section DE, a direct current source of auxiliary energy represented as a battery AB, and a circuit through which this source supplies exciting current to the transformer IT under the control of a contact l2 of the associated code following track relay TR.

As here represented, this impulse transformer IT has much in common with a corresponding device which is disclosed and claimed by a copending application Serial No. 218,923 filed July 13, 1938, by Charles B. Shields. In the present arrangement, however, the electrical constants of the impulse transformer and directly associated circuits are so chosen that each time that contact I2 interrupts the transformer exciting circuit the resulting rapid collapse of magnetic flux induces in the transformer secondary a pulse of auxiliary voltage having a potential substantially higher than that of track battery TB at the exit end of the section. The polarity of this pulse of induced transformer voltage, moreover, is such as to aid the main track circuit energy in holding the track relay TR picked up.

This relay TR preferably is of the polarized type. As long as the operating winding thereof remains deenergized, the contact l2 occupies the released position. When energy of the normal track circuit polarity is supplied to the winding, as during each energy on period of the main track circuit code from battery TB, the contact is picked up. When, however, energy of the opposite polarity is applied, the tendency merely is to move the contact to the extreme limit of its travel in the releasing direction.

In the form shown at location E, my improved exit end facilities comprise a detector relay KR which is connected in energy receiving relation with the rails of section DE, a circuit controlled by a contact of that relay for locally energizing the approach relay AR from a source designated by the terminals plus and minus, and means for making relay AR sufiiciently slow releasing to bridge the intervals between recurrent responses of the detector relay.

This detector relay KR is of the code following type and its operation winding is bridged directly across the rails to receive all potential differences appearing therebetween. The relay has special response characteristics which prevent it from operating on the voltage supplied by the track battery TB but yet which enable it to respond to the on code period pulses of higher voltage auxiliary energy received over the rails l and 2 from the impulse transformer IT at the opposite end of the track circuit. Preferably, moreover, relay KR is polarized in such a direction as to be insensitive to impulse energy which transformer IT supplies during the off code periods.

In operation of the complete approach control system shown in Fig. 1, the track rails I and 2 of section D--E act in the usual manner to transmit energy from one end of the section to the other as long as the section remains unoccupied. Each time, under such conditions, that contact 5 of the coding device GR is in the uppermost or on code period position, the track battery TB picks up the track relay TR over a circuit which may be traced from the positive terminal of the battery TB through a current limiting impedance [6, front contact 5 of device CR, conductors H and 18, track rail I, conductor 19, an impedance 2D, the winding of relay TR, conductor 2|, track rail 2 and conductors 22 and 23 back to the negative terminal of the battery TB. The detector relay KR also receives these recurring energy pulses of the main track circuit code but does not respond to them since their voltage is below the pick up value of that relay.

Each time that the coding contact 5 occupies the lowermost or off code period position, the track relay TR releases and contact l2 thereof completes for the impulse transformer IT the before mentioned exciting circuit which may be traced from the positive terminal of battery AB through conductor 25, the primary winding of the transformer, conductor 26 and back contact I2 back to the negative terminal of battery AB. The voltage induced in the transformer secondary due to the flux build up in the transformer core is of relatively reversed polarity and hence does not falsely operate the track relay TR or pick up the exit end detector relay KR.

In picking up at the beginning of each on code period contact l2 of track relay TR interrupts the just traced exciting circuit for the impulse transformer IT and thereby causes that transformer to impress between the track rails a pulse of induced auxiliary energy which is of sufiiciently high voltage to operate the relay KR at the exit end of the track section. The circuit over which this auxiliary energy is supplied may be traced from the left terminal of the secondary winding of transformer IT through conductors 28 and I9, track rail l, conductor l3, the winding of relay KR, an impedance 29, conductor 22, track rail 2 and conductors 2| and 30 back to the right terminal of the impulse transformer secondary.

Under the influence of each of these auxiliary pulses of high voltage energy supplied over the track rails, the code following detector relay KR picks up and thus completes, at contact l5, the local energizing circuit for the slow release approach relay AR. In the particular form shown, this energizing circuit extends from the positive terminal of the local supply source, through front contact [5 of relay KR, conductor 32, and the operating winding of relay AR back to the negative supply terminal.

These recurring pulses of energization cause the slow release approach relay AR to hold its contacts 9 and H continuously picked up, thereby maintaining the wayside signal Se and the track transformer TT at location E deenergized as long as the track section D--E remains vacant. The referred to slow releasing characteristics may, of course, be imparted to relay AR in any one of a number of well known manners, such as by internal design expedients or the use ofa snubbing impedance (not shown).

The purpose of the impedance 20 in the circuit of the track relay TR at the entrance location D is to reduce the amount of auxiliary energy from transformer IT which is diverted by the winding of the named relay from the main track circuit path l92l. At the exit location E, impedance It performs a comparable function in connection with the auxiliary energy which operates the detector relay KR in that it reduces the amount of diversion over the front point of contact 5 and the coding supply circuit of which battery TB forms a part of the total auxiliary energy which is received from the track rails. Likewise, the impedance 29 in the circuit of relay KR has a comparable eifeot of reducing the diversion of energy from track battery TB through the winding of relay KR and away from the rails I and 2 of the supplied track section.

In the event that a train comes into the track section D-E, the usual shunting action of its wheels and axles reduces to a very low value the potential difference between the rails I and 2 and thus deprives both of the relays TR and KR of pick-up energy. Relay KR now remaining continuously released, contact l5 thereof breaks the energizing circuit for relay AR. Deprived of the recurring energy pulses, that slow release relay completes, at contacts 9 and H, the energizing circuits for signal Se and transformer TT at location E.

In consequence, the named wayside signal lights the particular lamp selected by the decod ing equipment In and transformer TT-supplies the track rails with a. pulse of alternating current energy from source B-C each time that the contact 5 of device CR occupies its lowermost position. This energy is, as has been mentioned, suitable for the control of cab signals which may be carried by the train now proceeding through the section D-E and the circuit over which it is supplied may be traced from the right terminal of the secondary winding of transformeri TT through conductor 34, back contact 5, conductors I1 and I8, rail I, the wheels and axles (not shown) of the train, rail 2 and conductors 22 and 23 back to the left terminal of the track transformer secondary.

As soon as the rear of the departing train clears the exit end location E, the coded energy from track battery TB is again transmitted by the rails to the track relay TR at the entrance location D. That relay once more responds and the operation of contact l2 thereof causes impulse transformer IT again to supply the track rails with pulses of the high voltage auxiliary energy. These in turn are transmitted to detector relay KR at the exit location E and in responding to them its contact l5 again recurrently completes the energizing circuit for approach relay AR. That relay picks up and due to its slow release characteristics continuously deenergizes the wayside signal Se and the track transformer TT at location E, thereby restoring both of these devices to their normally inactive state.

From the foregoing it will have become evident that concurrently with the operation of the standard coded track circuit for signal control I have provided the non-line-wire approach control by employing the s0-called impulse principle of superimposed track circuit operation. That is, the standard coded track circuit is operated in the normal manner with the coded energy fed thereto at the exit end of the block. The impulse track circuit energy, however, is fed into the circuit in the reverse direction or at the entrance end of the block and it effects the necessary energization of the approach detector relay KR at the exit end as long as the block remains vacant.

Further regarding this code following detector relay KR, the critical or minimum operating voltage therefor may be such that while the relay will never pick up from the local track battery TB yet it may delay its release until the contact 5 of the local coding device CR disconnects the battery from the track rails. Moreover, since the polarity of the high voltage auxiliary energy pulses from the entrance end impulse transformer IT is the same as is that of the main coding voltage which the track rails receive from the battery TB, this code following detector relay need receive from the impulse transformer only sufficient more energy over that already obtained from the local track battery to cause the relay to pick up. In this manner relatively high sensitivity is at all times assured.

Referring now to 2, I have here represented a somewhat modified form of entrance end facilities which correspond to those shown at location D in Fig. 1. Interposed between the code following track relay TR and the impulse transformer IT is a repeater relay F? of the code following type which is energized over a front contact 36 of the relay TR from a suitable supply source designated by the terminals plus and minus. In this modification, the decoding apparatus In may be operated either by the repeater relay FP as shown or directly from the track relay TR as represented in Fig. 1.

This repeater relay F1? is of the code following type and carries the contact 12 over which exciting current from battery AB is supplied to the primary of the impulse transformer IT. Instead of being directly connected across the track rails i and 2 as in the system of Fig. 1 the secondary winding of this transformer is provided with an output circuit which includes a front contact 31 of the relay TR.

The purpose of the supplemental expedients just described as including the: repeater relay FF is to conserve the amount of track circuit energy which the. secondary winding of the impulse transformer IT diverts from the operating winding of the track relay TR at the beginning of each received pulse of main track circuit code energy from the track battery TB at location E (not shown in Fig. 2). From the diagram of Fig. 2, it will be seen that when contact 31 of relay TR is released, the secondary of the impulse transformer IT is disconnected from the track rails and hence the full complement of the track circuit pulse is made available for picking up the track relay TR.

Once picked up, contact 31 of relay TR restores the impulse transformer secondary connection and contact 36 picks up the code following repeater relay FP over a circuit extending from the positive supply terminal, through front contact 36, conductor 38 and the winding of relay FP back to the negative supply terminal. In picking up contact 12, relay FP now interrupts the exciting circuit for the impulse transformer and causes to be induced in the secondary winding thereof the before described pulse of high voltage auxiliary energy. The circuit over which this pulse is transmitted to the track rails may be traced from the left terminal of the transformer secondary through conductor 40, front contact 31 of relay TR, conductors 4| and I9, track rail I, the receiving apparatus (not shown) at the exit end of the track section, track rail 2 and conductors 2| and 42 back to the right terminal of the transformer secondary.

At the end of each on period of the main track circuit code, relay TR releases in the usual manner and deenergizes repeater relay FP which, in turn, releases contact 12 to complete the exciting circuit for the impulse transformer IT. Upon beginning of the succeeding on period of the track circuit code, the cycle of operations just described repeats itself and. in this manner the functions described in connection with the entrance end facilities shown at location D in Fig. 1 are performed by the modified apparatus of Fig. 2, with the added advantage that the secondary Winding of impulse transformer IT is disconnected from the track rails at the beginning of each on period.

Referring next to Fig. 3, I have there shown cut section facilities suitable for use with the approach control systems of Figs. 1 and 2. Such facilities are used when the main signal block length of track (D--E of Fig. 1) is, because of excessive length or for other reasons, divided into two track circuit sections by insulated rail joints 3 located as at Do in Fig. 3.

In the form shown at Do, these out section facilities are arranged to perform three functions. First, they repeat around the insulated joints 3 at the cut and into the rails of the section to the rear thereof the coded track circuit energy which is received from the rails of the forward section. Effecting this first function is a code following track relay TR which is operated by energy received from the forward section rails, a track battery TB which serves as an energizing source for the rear section rails and a coding contact 5 carried by relay TR which completes the rear section supply circuit each time that the relay is picked up. As in the case of the corresponding facilities at location E in Fig. 1, the rear section coding circuit of Fig. 3 may be traced from the positive terminal of battery TB through impedance l6, front contact 5 of relay TR, conductors l1 and 18, the track rails l and 2 and the conductors 22 and 23 back to the negative terminal of battery TB.

The second function performed by the cut section facilities of Fig. 3 is to repeat the pulses of high voltage auxiliary energy from the rear section location D (not shown in Fig. 3) around the Da joints 3 and into the rails of the forward section. Participating in this second function is a code following detector relay KR corresponding to the device of similar identification which has been described in connection with location E of Fig. 1, a source of auxiliary energy AB for the forward section, and an associated impulse transformer IT which receives exciting current from source AB. Upon the response of relay KR to each pulse of rear section auxiliary energy, contact l2 thereof interrupts the exciting circuit for transformer IT and causes its secondary winding to supply the forward section rails with a corresponding pulse of high voltage auxiliary energy. The output circuit for transformer IT corresponds to that shown in Fig. 2 and may be traced from the left terminal of the impulse transformer secondary through front contact 31 of relay TR, conductors il and IS, the track rails I and 2 and conductors 2| and 42 back to the right terminal of the transformer secondary.

The third function performed by the cut section facilities of Fig. 3 is to supply the rails of the rear section with coded alternating current energy suitable for cab signal control whenever a train enters that rear section. Aiding in this third function is a track transformer TT, a supply circuit therefor including terminals B and C and a controlling contact ll of a slow release approach relay AR, and means including a contact 15 of relay KR for energizing relay AR in step with the pulses of auxiliary energy which are received from the rear section rails. As long as these pulses continue to be received, relay AR is in this manner periodically energized and contact I l remains picked up to maintain transformer TT inactive. When, however, a train comes into the rear section relay KR continuously releases, relay AR is continuously deenergizecl and contact H thereof now releases to connect transformer 'IT with the the operation either of the main signaling scheme or of the improved approach control system of my invention which is used therewith.

As shown in Fig. 3, the relative polarities of the two there adjoining track circuits are staggered for the purpose of providing the usual protection against broken down rail joints. It will be understood, of course, that this staggering is not essential and is merely optional. The cut section facilities are capable of operating when the track circuit polarities either are or are not staggered.

From the foregoing it will be seen that I have made important improvements in that form of approach control without line wire scheme wherein the rails of each signal block length of track transmit code step pulses of auxiliary energy forwardly from the block entrance to effect the energization of a slow release approach relay at the block exit as long as the block remains vacant.

In particular, I have provided improved means for supplying the auxiliary energy pulses to the entrance end of each track circuit without interfering with the normal code following operation of the signaling system track relay there located; I have provided improved means forenergizing the slow release approach relay at the exit end of each track circuit in step with the pulses of this auxiliary energy which are there received; I have rendered this approach relay unresponsive to coded signal control energy which facilities of the signaling system supply to the track circuit at the same location; and I have provided cut section facilities which are suitable for use with approach control schemes of the improved type herein disclosed.

As all of these improvements are entirely a function of the track circuit and apparatus which is directly associated therewith, they are independent of the coding and decoding facilities of the coded signaling system and hence are usable with a wide variety of different types and forms of such facilities.

While I have explained my invention in an application wherein both the lamps of the wayside signal and the supply of alternating current cab signal energy are approach controlled, it will be understood that either one of these functions may be performed separately and that other comparable functions may also be provided for in my new system.

Although I have herein shown and described only a few forms of approach control apparatus embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination with a section of railway track, means for supplying the rails of said section with coded direct current energy of a given polarity, a code following track relay operated by said given polarity energy received from said rails but unresponsive to energy of the opposite polarity, a transformer connected across the section rails and adapted to generate impulse voltages which are substantially higher than the voltage of said coded track circuit energy, a direct current exciting circuit for said transformer, and means governed by said track relay for controlling said circuit in such manner that the transformer produces an impulse of output voltage having the said given polarity during each on period of the received trackway code.

2. In combination with a section of railway track, means for supplying coded direct current energy of a given polarity to the rails of said section, a code following track relay operated by said given polarity energy received from said rails but unresponsive to energy of the opposite polarity, atransformer connected across the section rails and adapted to generate impulse voltages which are substantially higher than the voltage of said coded track circuit ener y, a direct current energizing circuit for said transformer, and a contact of said track relay included in said circuit and acting to control it in such manner that the transformer produces an output voltage having said given polarity during each on period of the received trackway code and having said opposite polarity during each off period of that code.

3. In a system for approach controlling a signaling function without the use of line Wires, the combination with a section of railway track of: (1) exit end facilities comprising means for supplying coded energy to the rails of the section and a code following detector relay bridged across said rails and responsive only to voltages which are substantially higher than that of said coded energy; and (2) entrance end facilities comprising a code following track relay operated by energy received from said section rails, an impulse transformer having an output winding connected across said rails and adapted to generate voltages of said substantially higher value, a front contact of said track relay included in the output circuit of said transformer to disconnect said winding from said rails each time that that relay releases, a direct current exciting circuit for said transformer, and means controlled by said track relay for interrupting said exciting circuit each time that that relay picks up.

4. Approach control apparatus for use with a section of railway track which forms a part of a signaling system of the coded track circuit class comprising a code following track relay operated by energy received from the rails of said section, a transformer for supplying pulses of auxiliary energy to said rails, a circuit connecting the output winding of said transformer across the rails, means controlled by said track relay for completing said connection only when said track relay is fully picked up, a direct current exciting circuit for said transformer, and means controlled by said track relay for interrupting said exciting circuit immediately following each pick-up operation of the track relay.

5. In combination, a section of railway track,

means for supplying coded energy to the rails of said section, a code following track relay operated by energy received from said rails, a transformer for generating impulse voltages, a circuit connecting the output winding of said transformer across the section rails, means controlled by said track relay for completing said connection only when said track relay is picked up, a direct current exciting circuit for said transformer, and means controlled by said track relay for interrupting said exciting circuit immediately following each pickup operation of the track relay.

6. In combination, a section of railway track, means for supplying coded energy to the rails of said section, a code following track relay operated by energy received from said rails, a transformer for generating impulse voltages, a circuit including a front contact of said track relay for connecting the output winding of said transformer across said rails each time that the track relay picks up, a repeater relay energized over a front contact of said track relay, a direct current exciting circuit for said transformer, and a contact of said repeater relay included in said exciting circuit for the purpose of interrupting it each time that the repeater relay picks up.

'7. In combination, a section of railway track, means at the exit end of said section for supplying the rails thereof with signal code energy in the form of recurring on period pulses that are separated by off period intervals, a code following track relay located at the entrance end of said section and operated by the said pulses of signal code energy which are there received over said rails, means controlled by said track relay for further supplying the section rails with auxiliary energy in the form of pulses which occur during the on periods of said received signal code and which have a voltage substantially higher than that of said signal code energy, a code following detector relay at said section exit which from said rails receives said pulses of low voltage signal code energy at all times and also said pulses of higher voltage auxihary energy under vacant section conditions and which is operated by the combined energy of said signal code and auxiliary pulses but not by the energy of said signal code pulses alone, and traffic governing apparatus controlled by said code following detector relay and selectively responsive according as that relay is or is not operating.

8. In combination, a section of railway track, means at the exit end of said section for supplying the rails thereof with signal code energy in the form of recurring on period pulses that are separated by off period intervals, means located at the entrance end of said section and Operated by the there received pulses of said signal code energy for further supplying the section rails with auxiliary energy in the form of pulses which occur during the on periods of said signal code and which have a voltage substantially higher than that of said signal code energy, a code following detector relay at said section exit which from said rails receives said pulses of low voltage signal code energy at all times and also said pulses of higher voltage auxiliary energy under vacant section conditions and which is operated by the combined energy of said signal code and auxiliary pulses but not by the energy of said signal code pulses alone, a slow release approach relay energized when and only when said detector relay is operating whereby to be maintained picked up when and only when said track section is vacant, and traflic governing apparatus controlled by said approach relay and selectively responsive according as that relay is picked up or released.

CRAWFORD E. STAPLES. 

